Abscisic acid (ABA) has been the focus of intense investigation since it was identified in the 1960s as an endogenous small molecule growth inhibitor and regulator of plant stress physiology (K. Ohkuma, J. L. Lyon, F. T. Addicott, O. E. Smith, Science 142, 1592 (1963); C. F. Eagles, P. E. Wareing, Physiologia Plantarum 17, 697 (1964); J. W. Cornforth, B. V. Milborrow, G. Ryback, Nature 206, 715 (1965); J. W. Cornforth, B. V. Milborrow, G. Ryback, P. F. Wareing, Nature 205, 1269 (1965); D. Imber, M. Tal, Science 169 592 (1970)). Indeed, when one increases plant ABA sensitivity, improved drought and other stress tolerance results. See, e.g. Wang et al., Plant J. 43:413-424 (2005); Pei et al. Science 282:287-290 (1998); US Patent Publication No 2004/0010821. Genetic analyses have identified many factors involved in ABA signaling, including the type 2 C protein phosphatases (PP2Cs) ABI1, ABI2 and relatives that form the closely related ABI1/AHG1 clades that function as redundant negative regulators of ABA signaling (R. R. Finkelstein, S. S. L. Gampala, C. D. Rock, The Plant Cell 14, S15 (2002); P. McCourt, Annual Review of Plant Physiology and Plant Molecular Biology 50, 219 (1999); A. Schweighofer, H. Hirt, I. Meskiene, Trends in Plant Science 9, 236 (2004)). Several ABA binding proteins have been reported, however it is not clear how they regulate the myriad effects of ABA, because they do not appear to act through known regulators of its signaling pathway (X. Liu et al., Science 315, 1712 (Mar. 23, 2007); F. A. Razem, A. El-Kereamy, S. R. Abrams, R. D. Hill, Nature 439, 290 (2006); Y. Y. Shen et al., Nature 443, 823 (Oct. 19, 2006)). Additionally, the characterized receptors show negligible binding to the non-natural stereoisomer (−)-ABA 1 at concentrations ˜1000-fold higher than their Kds for (+)-ABA 2. (−)-ABA is bioactive in most ABA assays (B.-L. Lin, H.-J. Wang, J.-S. Wang, L. I. Zaharia, S. R. Abrams, Journal of Experimental Botany 56, 2935 (2005); D. Huang et al., The Plant Journal 50, 414 (2007)) and acts through the same signaling pathway as (+)-ABA (E. Nambara et al., Genetics 161, 1247 (July, 2002)), suggesting that receptors that recognize both (−) and (+)-ABA remain to be discovered.